A virtualenv is optional. If you are installing Booktype on a machine running lots of other Python programs (such as your desktop machine) it would be beneficial, to prevent conflicting versions of Python dependencies being installed for some other program. If you are installing on a server dedicated to running Booktype it is probably not necessary.

As for the -p argument to the virtualenv command, that might be required if Python 2.7 is not the default on your machine. On GNU/Linux, the python command is usually a symlink to /usr/bin/python2.7 but that may not be the case on your Mac.

While installing pip, you might as well upgrade your copy of setuptools:

easy_install -U setuptools

Please see the GNU/Linux install guide for more tips, as this is better tested than the OS X instructions: